1. Field of Invention
This invention relates to a video signal processing circuit, which performed clamping and auto gain control (AGC) to a video signal, especially to a video signal processing circuit suitable to process a video signal, which includes a copy guard signal and a pseudo horizontal synchronization signal.
2. Description of Related Art
Home VTR is used to view videotapes, on which video soft is recorded. Some of the videotapes sold in the market have the video signal with a copy guard signal for preventing the duplication of the recorded video soft.
FIG. 2 shows a waveform of the video signal with the copy guard signal. FIG. 2 shows a composite video signal including the copy guard signal with an amplitude to cover from white to black levels and a pseudo horizontal synchronization signal, a vertical synchronization signal, a horizontal synchronization signal, and an equalizing pulse. Various kinds of signal-processing are performed to the video signal reproduced from the videotape.
It is necessary to equalize the edge level of the horizontal synchronization signal by using a clamp circuit and to stabilize the direct current level before performing the signal processing. Also, a AGC circuit equalizes the level of the video signal. The clamp circuit clamps the edge level of the horizontal synchronization signal. This requires a clamp pulse with a pulse width slightly shorter than the pulse width of the horizontal synchronization signal. The AGC circuit also detects the direct current level in a back porch portion of the horizontal synchronization signal. This requires a stretch sync pulse with a pulse width slightly longer than the pulse width of the horizontal synchronization signal.
FIGS. 3A-3E show waveforms of the conventional video signal processing.
FIG. 3A is an enlarged figure showing the waveform of the copy guard signal shown in FIG. 2. FIG. 3A indicates that the copy guard signal is included in the normal horizontal synchronization signal. The cycle of the horizontal synchronization signal is one horizontal period as shown in the figure. The copy guard signal has a pseudo horizontal synchronization signal with the cycle shorter than that of the horizontal synchronization signal and an AGC pulse. Although the number of the cycles varies among the devises, the copy guard signal has, for example, four cycles. The pulse width of the pseudo horizontal synchronization signal is less than a half of the pulse width of the horizontal synchronization signal. The AGC pulse has amplitude of from white to black.
The signal shown in FIG. 3A is sliced at a slicing level shown as the dotted line in FIG. 3A separating the horizontal synchronization signal portion as the pulse signal b-1 shown in FIG. 3B. The clamp pulse is generated as shown in FIG. 3C based on the pulse signal b-1. The edge level of the horizontal synchronization signal shown in FIG. 3A can be detected by using the clamp pulse.
A stretch sync pulse shown in FIG. 3D is also generated based on the pulse signal b-1 in FIG. 3B. The level of the back porch portion shown in FIG. 3A can be detected by using the stretch sync pulse.
Therefore, it is possible to perform clamping as well as AGC to the video signal reproduced from the videotape when a plurality of pulses shown in FIGS. 3A-3E are generated.
However, there are some problems during a copy guard signal period when the clamp pulse shown in FIG. 3C and the stretch sync pulse shown in FIG. 3D are used. The pseudo horizontal synchronization signal with the short pulse width generated during the copy guard signal period is synchronically separated like the normal horizontal synchronization signal is separated. If the clamp pulse based on the pseudo horizontal synchronization signal is used for clamping, the clamp can not be performed properly.
The signal shown in FIG. 3B passes through a low pass filter, LPF, and the phase of the signal will be slightly deviated toward right against the signal shown in FIG. 3A. Therefore, since the AGC pulse comes right after the pseudo horizontal synchronization signal during the copy guard signal period, the edge level (white signal) of the AGC pulse is detected by mistake. A mask pulse shown in FIG. 3E is used in the prior arts in order to block the clamp pulse produced from the copy guard signal.
However, the standard of the copy guard signal is changed often, and the timing of the mask pulse should be changed accordingly. There are also some copy guard signals that do not follow the standardization.
Additionally, if the mask signal is used, the clamping will not be performed even to the normal horizontal synchronization signal during the period when the copy guard signal exists. The change in the direct current level is large during the copy guard signal period because the size of the brilliant signal constantly changes. When this sort of signal goes through a capacitor for stopping a direct current, a phenomenon called a V-sag, where the average direct current level of the video signal changes, will take place. Although a keyed clamp can absorb the V-sag, the V-sag can not be absorbed when the clamp pulse is stopped during the copy guard period.
The AGC suppression of an analog signal takes place in a digital video image device such as DVD recorder when the AGC suppression is not favorable. Therefore, the signal processing circuit that lets the horizontal synchronization signal pass and that blocks the pseudo horizontal synchronization signal during the copy guard signal period is preferable.